Direct methanation of coal, in order to produce synthesis natural gas, has been the object of much effort for several years. For example, a process is known wherein powdered coal is reacted with overheated hydrogen (see for example U.S. Pat. Nos. 3,960,700 and 4,206,032).
These processes suffer from the disadvantages due to the difficulty of uniformly distributing the powdered solid coal into the numerous nozzles required for operating the process on an industrial scale and to the high hydrogen consumption.
According to another proposal, particles of coal or pitch are used as heat carrier and as reactant for producing a hydrogenating gas.
The U.S. Pat. No. 3,855,070 describes a process of this type making use of the larger coke particles as reactant for producing hydrogenating gas: these particles are treated with a mixture of air or oxygen and steam. The resultant gas is then burnt in contact with fine particles of coke, so as to heat them, the heated fine particles being then fed to the coal pyrolysis zone where they supply the required heat. This process suffers from disadvantages inasmuch as it is particularly difficult to completely gasify large particles in view of the limited diffusion therethrough.
According to another proposal (U.S. Pat. No. 4,162,959), the coke discharged from the pyrolysis zone is successively subjected to a partial combustion, destined to increase its temperature, then to a treatment with steam destined to produce the hydrogen necessary for the reaction; the residual coke is then fed to the pyrolysis zone to supply the heat required for the reaction. This process results in a certain simplification of the technique of U.S. Pat. No. 3,855,070, but hs some defects. As a matter of fact, the treatment with steam being endothermic, the solid issued from this treatment cannot be brought to a very high temperature, for example of 1000.degree. C., which is detrimental to a good development of the pyrolysis.